Thin provisioning of a logical unit of storage (LUN) involves on-demand allocation of physical storage blocks to the LUN from a common pool of physical storage blocks. In contrast to traditional (or fat) LUN provisioning in which all of the physical storage is allocated to the LUN up front, thin provisioning allocates physical storage as needed. Accordingly, a thinly provisioned LUN typically appears larger (i.e., provides a larger available address space) than the amount of physical storage that actually has been allocated to that LUN.
In a conventional data storage system that uses 8 KB as a standard block size, suppose that a storage processor is tasked with writing 4 KB of host data from a host computer to a thinly provisioned LUN. First, the backend resources allocate an 8 KB physical block of storage to the thinly provisioned LUN from a pool of free physical storage blocks. Next, the storage processor forms a full block of data in memory by combining the 4 KB of host data and 4 KB of zeroes, and then writing the full block of data from the memory to the allocated 8 KB physical block. Such overwriting of the unused portion of the allocated 8 KB physical block with zeroes avoids any possibility of data leakage since the allocated 8 KB physical block may have been used previously to hold other host data, i.e., that 8 KB physical block may have been recycled.